1. Field of the Invention
The present invention relates generally to a signal regenerator for use in an optical communication system, and particularly to such a regenerator that combines dispersion compensation with the saturable absorption properties of a nonlinear optical loop mirror or a nonlinear amplifying loop mirror.
2. Technical Background
The concept of dispersion management emerged early in the development stages of optical waveguide systems. Dispersion management is enabled by the property of waveguides fibers that allows adjustment of the waveguide dispersion by alteration of the optical waveguide fiber refractive index profile. In particular, the waveguide dispersion can be made to substantially cancel, i.e., subtract from, the material dispersion to provide a waveguide fiber having a total dispersion near zero over an extended wavelength range. The total dispersion of a waveguide fiber (also called the chromatic dispersion or the group velocity dispersion) is the algebraic sum of the waveguide dispersion and the material dispersion of the optical waveguide fiber. The convention in the art is to assign a positive value to total dispersion if the total dispersion causes light of shorter wavelength to travel at a higher speed in the fiber in comparison to the speed of longer wavelength light. Conversely; negative total dispersion causes light of longer wavelength to travel at higher speed in the fiber.
The waveguide dispersion can be altered to provide waveguide fibers that have a zero dispersion wavelength at any point in a wide wavelength range. For example zero dispersion wavelength of a waveguide fiber can be placed anywhere in the range from 200 nm to 2000 nm. In addition, the slope of the total dispersion can be made positive or negative essentially independently of the placement of the zero dispersion wavelength. These capabilities allow dispersion compensation to be achieved by altering the sign of the total dispersion along the length of an individual waveguide fiber. Further, dispersion compensation can be achieved on an overall system basis by forming the system of positive and negative waveguide fibers. The accumulated dispersion of a system is determined by adding the dispersion products of the waveguides that make up the system length and dividing by the total system length. The dispersion product of a waveguide fiber is defined as the total dispersion of the waveguide fiber in ps/nm-km multiplied by the length of the fiber.
As a valuable adjunct to dispersion management is the optical amplifier, which is used to manage attenuation. Dispersion management combined with optical amplification raises the possibility of a dispersion and attenuation free system, having repeater spacing limited only by spontaneous noise from the amplifiers, frequency chirping of the signal source, and non-linear optical effects.
A signal regenerator module that, in addition to compensating dispersion and attenuation, also removed spontaneous noise, pulse timing jitter, and reduced or eliminated non-linear effects would serve to greatly decrease system cost by preserving signal pulse integrity in systems having larger regenerator spacing than is possible with present systems using standard regenerators.
Recent theoretical and experimental (PCT WO 98/36512; Golovchenko et al, Electronics Letters, v. 33, n. 9, p. 73, (1997); Nakawaw, et al, IEEE Photonics Technology Letters, v. 8, n. 8, p. 1088, 1996; F. Favre. Et al, Electronic Letters, v. 33, n. 25, p. 2135, (1997); T. Yu, Optics Letters, v. 22, n. 11, p. 793, (1997)) work in dispersion managed systems employing a return-to-zero (RZ) format for soliton signals has shown that allowing the RZ soliton signals to alternately broaden in highly dispersive optical waveguide fiber of one sign and then contract in highly dispersive optical waveguide fiber of the other sign has a beneficial impact on overall signal integrity. For example, the RZ soliton signals could broaden in positive dispersion optical fiber and then contract in negative dispersion optical fiber. In particular in varying-soliton signals, timing jitter due to amplified spontaneous emission (ASE) is reduced, signal-to-noise-ratio in the receiver (SNR) is improved, the impact of discrete amplifier power perturbations is reduced, and the deleterious effects of signal collisions or multi-wavelength signal interactions is reduced.
An opportunity therefore exists to exploit this advantageous combination in systems using an RZ format. Furthermore, the combination can be configured to further enhance the beneficial effect on the varying-soliton signals.
Throughout this application the term varying-soliton(s) is used to describe RZ (return to zero) signal pulse(s), i.e., soliton pulses whose amplitude, width, or shape are caused to vary along at least a portion of the waveguide fiber, in particular along the waveguide fibers of the dispersion compensating optical regenerator in accord with the invention. This designation of varying-soliton distinguishes the signal pulses of the present application from those that propagate in systems designed to maintain ideal, i.e., invariant, soliton signals. It also distinguishes the signal pulses of the present application from non-return-to-zero pulses used in commercial systems today.
One aspect of the present invention is a dispersion compensating optical regenerator for use in a waveguide fiber telecommunications system. This passive optical component combines the functions of dispersion compensation with signal regeneration, where signal regeneration includes recovering signal amplitude and shape. The dispersion compensating optical regenerator system comprises a positive total dispersion waveguide fiber for the transmission fiber and a negative total dispersion waveguide fiber for dispersion compensation, where the negative dispersion waveguide fiber is a part of a non-linear optical loop mirror (NOLM) or a non-linear amplifying loop mirror (NALM). The respective positive and negative total dispersion waveguide fibers have respective lengths and total dispersion magnitudes selected to provide a pre-selected amount of dispersion compensation. In particular, the waveguide fiber dispersion products, i.e., the product obtained by multiplying fiber total dispersion by fiber length, of the respective waveguide fibers are added algebraically. The respective fiber lengths and dispersions are chosen such that the magnitude of the algebraic sum is made to fall within a desired range.
An advantageous range is one that does not include zero, thereby limiting resonant non-linear phenomenon of four wave mixing. Another advantageous range choice is one in which the algebraic sum is positive, thereby allowing for formation and propagation of soliton pulses. The invention is particularly suited for use in varying-soliton transmission as will be pointed out in the detailed description below. The upper limit of the algebraic sum should be small to limit dispersion power penalty. Thus a preferred range of total average dispersion of the system is 0.01 to 5.0 ps/nm-km and a more preferred range is 0.1 to 1 ps/nm-km.
In an embodiment of this first aspect of the invention, the phase shifting means is an asymmetrical coupler that divides the signal pulses into counter-propagating pulses having different amplitude. The higher amplitude pulses corresponding to one of the propagation directions will undergo a larger phase shift due to self phase modulation.
In another embodiment, the respective amplitudes of the counter-propagating pulses are made asymmetric by an optical amplifier asymmetrically placed along the length of the optical waveguide fiber comprising the loop mirror. An alternative statement of the asymmetric placement of the optical amplifier is that the length of the fiber of the loop mirror coupled to one port of the amplifier is different from the length coupled to the other port of the amplifier. In this case the optical amplifier has two ports, one for signal input and one for signal output, where the ports are symmetrical in that either port can be the input or output port. The optical amplifier can be selected to amplify the varying-soliton signals coupled out of the loop mirror to an extent that the insertion loss due to the loop mirror (signal power lost in traversing the loop mirror) is compensated. This selection of amplifier obviates the need for a post-loop-mirror-amplifier sometimes used to compensate loop mirror insertion loss.
In either of the above embodiments, a polarization controller coupled in series with the waveguide fiber of the loop mirror can be used to insure that low intensities come out the input of the central coupler, i.e., are reflected, while high intensities come out the output of the central couple, i.e., are transmitted. A polarization controller is known to produce a linear phase shift in signals passing therethrough such that it controls the fiber port through which the signal exits the loop.
In any of the preceding embodiments, a phase shift of 180 degrees (xcfx80 radians) between the counter-propagating signals is desirable, because the 180 degree phase shift insures maximum signal power is coupled out of the loop mirror.
In a further embodiment of this first aspect of the invention the phase shifting means can be selected from the group consisting of an asymmetrical coupler or an asymmetrically placed optical amplifier, either a discrete erbium-doped fiber amplifier (EDFA) or a Raman optical amplifier. Any combination of these phase shifting means can be used to obtained the desired phase shift, which in most systems is a 180 degree (xcfx80 radians) phase shift. For example, an EDFA could be used together with an asymmetrical coupler, where the amplifier could be used to compensate for too high or two low an asymmetric power division of the signals by the coupler.
Asymmetric Raman amplification is created in the fiber loop by coupling pump light into the loop fiber in only one direction near the central coupler. Amplification is highest where the pump power is greatest. In fact, Raman gain in units of decibels is directly proportional to pump power in linear units of Watts.
In an embodiment of this first aspect of the invention, the optical waveguide fiber of the loop mirror is selected to have a negative total dispersion in the range of xe2x88x9270 ps/nm-km to xe2x88x92100 ps/nm-km. Preferably the negative dispersion waveguide fiber of the loop mirror has an effective area in the range 20 xcexcm2 to 40 xcexcm2 and a non-linear refractive index not less than 2xc3x9710xe2x88x9220 m2/Watt, where the watt refers to the signal pulse power. More preferably the negative total dispersion of the fiber of the loop mirror is in the range xe2x88x9280 ps/nm-km to xe2x88x9290 ps/nm-km, the effective area is in the range 20 xcexcm2 to 30 xcexcm2, and the non-linear refractive index is in the range 2.5 m2/W to 4 m2/W.
A further embodiment of the invention includes an optical preamplifier coupled to the regenerator to increase the amplitude of signals propagating from the preamplifier to the loop mirror such that the signal amplitude in the loop mirror is above the threshold amplitude at which the signal undergoes self phase modulation.
In another aspect, the present invention is an optical waveguide telecommunication system which includes a signal transmitter optically coupled to a signal receiver by means of an optical waveguide fiber. Incorporated in series arrangement into the waveguide fiber is a dispersion compensating optical regenerator as set forth in the first aspect of the invention. The telecommunications system is particularly suited to varying-soliton pulse propagation because of the features of the dispersion compensating optical regenerator.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.